Agricultural soils contain thousands of weed seeds per square foot. The density of weed seeds in the weed seedbank is influenced by past farming practices and will vary from field to field. This definition explains the meaning of Marijuana Seed Bank and why it matters. Weed Seedbanks The weed seedbank in ƒi in the current period, then becomes equal to those surviving seeds that do not germinate in the seedbank at t−1, sbi,t−1, and the contribution of seeds
Agricultural soils contain thousands of weed seeds per square foot. The density of weed seeds in the weed seedbank is influenced by past farming practices and will vary from field to field (Table 1, Renner, 1999) and even between areas within fields. In intensively cropped fields in the north central corn belt, the weed seedbank ranged from 56 – 14,864 seeds per square foot (Forcella et al., 1992).
Table 1. How many weed seeds are in the weed seedbank?
Number of seeds per square foot
W.K. Kellogg Biological Station—Long term Ecological Research Site, Hickory Corners, Michigan
Composition of Weed Seedbanks
Seedbanks are made up of numerous weed species although only a few species will comprise 70 to 90 percent of the total seedbank. Common lambsquarters (Chenopodium album) is the dominant weed seed in many field soils in the north central region of the United States, including Michigan. Common lambsquarters dominated the weed seedbank in three of five cropping systems at the Long Term Ecological Research (LTER) site at the Kellogg Biological Station in Michigan (Figure 1).
Weed Seed Distribution in Soil
The location of seeds in the weed seedbank is influenced by the tillage system. More weed seeds will remain near the soil surface when tillage is reduced or no-till farming is practiced (Figure 2). These changes in the distribution of the weed seeds in the weed seedbank will influence weed emergence and the resulting weed population in farm fields.
Sources of Weed Seed
Weed seeds can reach the soil and become pat of the seedbank through several avenues. The main source of weed seed in the seedbank is from, weeds that matured in the field and set seed. Annual weeds produce large numbers of seeds (Table 2, Renner, 1999). Weed seed can also enter the seedbank by wind, water, animals, birds, and human activity. Some weed seeds (such as dandelion) are wind-dispered. Weed seeds can reach a field site following flooding of drainage ditches or adjacent rivers. Wildlife and livestock can spread weed seed, either directly or by spreading of manure. Farming operations can add weed seed to the seedbank by moving soil that contains weed seeds on farm equipment or moving weed seeds to other fields when harvesting crops.
Table 2. Typical Michigan weed seed production
Number of seeds per plant
Weed density (per 33 feet of crop row)
Weed Seed Fate
A weed seed can have numerous fates once it is dispersed in a field (Figure 3, Renner, 1999). Some weed seeds will decay in the soil. Other seeds will decay in the soil. Other seeds will not decay but will no longer have the ability to germinate (the seeds are not viable). Some weed seeds will germinate and die, while other weed seeds will germinate and emerge. Some weed seed will be predated by various predators including birds, rodents, crickets, carabid (ground) beetles, and ants. Seed predation occurs mainly on or near the soil surface. Many weed seeds will remain dormant in the soil and not germinate regardless of environmental conditions. However, dormancy is not permanent and seeds of many weed species change from a state of dormancy to non-dormancy. This is called dormancy cycling (Figure 4, Renner, 1999). Seed dormancy is a survival mechanism and it is a major barrier to weed control in agroecosystems. Only a fraction of the weed seeds (less than 10 percent of most weed species) germinate each year. Therefore dormant seeds perpetuate the weed seedbank and weed populations in farm fields.
Weed Seed Persistence
Under agricultural conditions the average time that a weed seed will persist in soil and still be capable of germinating (remain visible) is less than five years. Some weed species will have more persistent seed than others. Velvetleaf (Abutilon theophrasti) and clovers (Trifolium sp.) have persistent seedbanks. Tillage also influences seed longevity in soil since weed seeds usually remain viable longer if they are buried. Seed on or near the soil surface is exposed to predators and seed decay which reduces seed persistence.
Managing the Weed Seedbank
The best way to manage the weed seedbank is to not allow weeds to set seed in the field. Over a six-year period in Colorado, common lambsquarters and redroot pigweed seeds were reduced to 6 and 1 percent, respectively, of the original seedbank in a continuous corn rotation where herbicides were applied and the fields cultivated (Schweizer and Zimdahl, 1984). In a Nebraska study, the broadleaf and grass weed seed density in soil declined by 95 percent over a five-year period. However in the sixth year weeds were not controlled and the weed seedbank increased to within 90 percent of the original level at two of five locations (Burnside et al., 1986). These studies illustrate two important points in weed seedbank management. First, there is a rapid decline in the weed seedbank when weeds are not allowed to set seed. Secondly, the few weed seeds remaining in the weed seedbank are capable of infesting the farm fields and returning the number of weed seeds in the seedbank to high levels. Therefore weeds must be managed every year to reduce the weed seedbank.
Other farming practices can influence the weed seedbank. Burying weed seed by tilling the soil increases longevity of weed seeds in the seedbank. Leaving weed seeds on the soil surface exposes weed seeds to predation which will reduce the number of weed seeds in the seedbank. Leaving weed seeds on or near the soil surface may increase the number of weed seeds that decay after being infected by fungi or other microorganisms. Livestock manure that is stored has fewer viable weed seeds compared to fresh manure. Cleaning tillage and harvest equipment can reduce the movement of weed seed from field to field.
Understanding weed seedbank dynamics is the first step in managing the weed seedbank. Reducing the number of weed seeds in the weed seedbank will improve our management of weeds in agroecosystems.
Burnside, O. C., R. G. Wilson, G. A. Wicks, F. W. Roeth, and R. S. moomaw. 1986. Weed seed decline and buildup under various corn management systems across Nebraska. Agronomy Journal. 78:451-454.
Forcella, F., R. G. Wilson, K. A. Harvey, D. A. Alm, D. D. Buhler, and J. Cardina. 1992. Weed seedbanks of the U.S. corn belt: magnitude, variation, emergence, and application. Weed Science. 40:636-644.
Renner, K. A. 1999. Weed ecology and management. Pages 51-68 in Michigan Field Crop Pest Ecology and Management. M. Cavigelli, ed. Michigan State University Extension Bulletin E-2704.
Schweizer, E. E. and R. L. Zimdahl. 1984. Weed seed decline in irrigated soil after six years of continuous corn and herbicides. Weed Science. 32:76-83.
Marijuana Seed Bank
A marijuana seed bank is a business which specializes in not only storing and selling cannabis seeds, but also on feminizing seeds to reduce the chance of male plants developing and maximize yields for commercial and individual growers.
Maximum Yield Explains Marijuana Seed Bank
As more and more states legalize marijuana, growers big and small find that they need to locate a reliable source of cannabis seeds. Generally, this is done through a marijuana seed bank – a company that develops cannabis strains, stores seeds, and then sells those seeds to commercial and individual growers. However, there’s more to it than that.
A marijuana seed bank is not just another grower, or a seed repository. These companies are at the forefront of seed feminization and strain development. Many seed banks have created highly-popular strains known for specific benefits or effects, such as providing greater mental clarity and focus, or for higher THC content.
Seed feminization is even more important, particularly for growers. Only female plants grow buds – male plants may have trace amounts of THC in their leaves, but they produce pollen, instead of THC-laden buds. Therefore, growers should focus primarily on growing female plants.
The problem is that when left to nature, seeds tend to produce a 50/50 split between male and female plants. Growers then spend time and resources growing plants, half of which must be discarded when it becomes apparent that they are male. Seed feminization reduces the percentage of male to female seeds by a significant amount.
Finally, a marijuana seed bank can also offer growers access to auto-flowering seeds. These plants grow to maturity quickly, in as little as eight weeks. The growing process is greatly simplified, and the yield is maximized, ensuring a better return on investment for growers.
The weed seedbank in ƒi in the current period, then becomes equal to those surviving seeds that do not germinate in the seedbank at t−1, sbi,t−1, and the contribution of seeds dispersed in the landscape in the current period, as shown in Eqs (5) and (6):(5)sb1,i,t=sb1,i,t−1×1−sm1×1−gr1+∑j=1nD1,j,i,tfjfi(6)sb2,i,t=sb2,i,t−1×1−sm2×1−gr2+∑j=1nD2,j,i,tfjfi
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The Future of Agricultural Landscapes, Part III
David A. Bohan , . Michael J.O. Pocock , in Advances in Ecological Research , 2021
188.8.131.52 Seedbank counts
France: The weed seedbank was assessed using the methods described in Heard et al. (2003) . Seedbank abundance was estimated by taking five soil cores (1.5 L in total between 0 and 20 cm depth) at the 4 and 32 m sampling points along the transects. The seedbank was estimated by germination of soil samples in a greenhouse under controlled conditions (18/15 °C day/night temperature regime with 12:12 h light:dark cycle). Counting and species identification of the germinated seeds in the samples were done up to 18 weeks after sample preparation. Weed seeds germinating from the soil samples collected in 2017 and 2018 were identified to species and summed to a total count of seeds per sampled field.
Seedbank diversity in Catalonia
Izquierdo et al. (2009) examined how the spatial distribution of weed seedbank diversity was affected by weed control. They examined the changing spatial distribution of weed seeds in an 8-ha winter wheat field (Triticum aestivum) field in western Catalonia from 2001 to 2003. The field was regularly treated with herbicides to control grass and broadleaf species, except only grass herbicides were administered in 2002 and 2003. 16-cm 2 soil cores were taken at 10-m intervals on a 150 × 150 m grid in the wheat field in January of each year. Seeds were allowed to germinate in a greenhouse, identified, and the density per square meter estimated for each species at 254 sample points (2 points were skipped). The distribution of weed seed diversity within the 2.25-ha area was mapped for each year. Izquierdo et al. found that the spatial distributions of Shannon diversity and evenness became increasingly patchy over time. Both grass and broadleaf weed patches moved and varied in size from year to year. In general patches of broadleaf weeds decreased in response to herbicide application, but the absence of a grass herbicide application in the first year enabled grass patches to expand contributing to increased patchiness. Izquierdo et al.’s Table 1 gives the density and SE of seeds (#/m 2 ) for 30 weed species. Despite the year-to-year variation in diversity and spatial distribution, the 3 years’ mixed-species TPLs do not differ significantly and are best described by a single line ( Fig. 6.8 ; Appendix 6.K).
Fig. 6.8 . There is no difference between years in the mixed-species TPL (NQ = 254, NB = 68) of a community of 30 weed species in the seedbank of a field in Catalonia. Each point in this graph is a different weed species recovered in sampling the seedbank by soil core.
Data from Table 1 in Izquierdo et al. (2009) .
Weedy rice (Oryza spp.)
Understanding factors influencing weedy rice seed decay in the soil can have important implications for management targeting weed seedbank of weedy rice. However, information on factors affecting the decay of weedy rice in DSR systems is meager. Soil moisture has been probably best documented for its influence on weedy rice seed decay. For example, winter flooding in Italy between rice crops reduced the viability of weedy rice seeds on the soil surface by >95% compared to a reduction by 26%–76% when the field is left dry ( Fogliatto et al., 2010 ). The study concluded that the reduction in viability was partly due to seed decay of nongerminated seeds under low-temperature conditions and flooding. However, another study conducted in Korea by Baek and Chung (2012) also observed that winter flooding reduced the germination of weedy rice but the effect was not as dramatic as reported by Fogliatto et al. (2010) . They observed that >60% weedy rice seeds could overwinter under flooded conditions, whereas in dry conditions, about 90% weedy rice seeds could overwinter.
Ecological weed management in Sub-Saharan Africa: Prospects and implications on other agroecosystem services
Paolo Bàrberi , in Advances in Agronomy , 2019
1.2.3 Reduced weed seedbank size
The major part of weeds in agricultural land reproduce and survive as seeds, thus the soil weed seedbank represents the main source of future weed infestations. Depletion of the weed seedbank can be obtained by increasing seed losses and/or reducing seed inputs. Losses can occur through seed predation, seed decay, and increased germination ( Gallandt, 2006 ).
Weed seed predation, especially after seeds have been shed on soil, may be an important determinant of seedbank losses ( Davis et al., 2013 ; Westerman et al., 2011 ). Insects and small rodents are the main contributors to weed seed predation, thus manipulation of agricultural habitats as to attract them (e.g., no-till, delayed stubble cultivation, introduction of uncultivated strips within fields or as field margins) is expected to increase the number of weed seeds predated ( Landis et al., 2005 ). Carabid beetles are among the most important consumers of weed seeds. It should be kept in mind that seed consumption by carabids is influenced by several factors, including weed species, seed physiological state, insects gender, activity-density level, and seed burial depth ( Kulkarni et al., 2015, 2016 ).
Weed seed decay is a mechanism so far poorly understood and consequently poorly exploited. It refers to the creation of soil conditions that are conducive to increased seed mortality through, e.g., fungal attack. Recently, some interesting results have been obtained by Gómez et al. (2014) , who nevertheless pointed out that are differences in weed species susceptibility to decay, indicating the need to develop species- and cropping system-specific management solutions.
Increased weed seed germination results in an output to the seedbank. This can be achieved, e.g., by the application of the false- and stale-seedbed techniques, i.e., the anticipated soil seedbed preparation which allows stimulation of germination and emergence of weed seedlings that are subsequently destroyed before the actual crop seeding or crop emergence takes place ( Cloutier et al., 2007 ). In the false seedbed technique seedling destruction usually occurs by harrowing or similar mechanical tools whereas in the case of the stale seedbed technique it occurs by chemical herbicides or by thermal methods (flame weeding or soil steaming), to avoid any further soil disturbance. Weed seed losses can also occur when seed germination is not followed by seedling emergence, usually because the seed is placed too deep down the soil and has not enough reserves in its endosperm to sustain seedling growth until it reaches the soil surface and becomes autotroph. This phenomenon is referred to as “fatal germination” ( Fenner and Thompson, 2005 ).
Weed seedbank replenishment can also be avoided by preventing production and shedding of new seeds. This can be obtained as an outcome of increased competition or as an effect of a well planned crop rotation ( Légère et al., 2011 ). However, it is also important to prevent seed shedding from late emerging weeds that, although usually unable to diminish crop yield in the same growing season, may create potential weed problems in subsequent crops or growing seasons through their seed inputs. Similarly, it is important to avoid weed seed shedding (e.g., by stubble cultivation or mowing) in the period between two crop growing cycles, an important issue that many farmers tend to disregard.
Integrated Weed Management in Organic Farming
Charles N. Merfield , in Organic Farming , 2019
5.4 Weed Seed Rain and Seedbank
As discussed in Section 5.2.4 , the core of weed management rests on minimizing the weed seed rain and therefore minimizing the weed seedbank. This is why managing the weed seed rain is the most important part of integrated organic weed management and must therefore be the first priority.
To illustrate the direct relationship between the weed seedbank and in-crop weeds, a study by Rahman et al. (1996) studying the number of emerged weeds vs. the number of viable weed seeds in the soil found a clear, almost one-to-one relationship ( Fig. 5.9 ). Clearly, the larger the weed seedbank, the larger the population of in-crop weeds.
Figure 5.9 . Number of weed seedlings emerged versus the number of viable seeds found in each of 48 individual soil samples. Note log scales both axes.
Adapted from Rahman et al. (1996) .
In terms of the linkage between weed seed rain and in-crop weed populations, Gallandt et al. (2010) found that by preventing weed seed rain they could reduce subsequent years’ weed seedbanks compared with other autumn treatments between 45% and 93% and weed seedling densities by 23% to 90%. In Western Australia preventing the seed rain of annual ryegrass ( Lolium rigidum) reduced in-crop ryegrass emergence by 90% in 4 years ( Walsh et al., 2013 ). Another perspective is illustrated by a study by Rahman et al. (1998) whereby they tilled soil monthly for 4 years, achieving an exponential decline in the weed seed bank, represented by four weed species, both monocotyledons and dicotyledons ( Fig. 5.10 ).
Figure 5.10 . Decline in seed numbers of four weed species at the Ruakura Research Centre site following monthly tillage over a period of 4 years. Note y-axis is a logarithmic scale.
Adapted from Rahman et al. (1998) .
While this level of tillage in real-world farming is clearly excessive and would be highly damaging to soil, it, along with the other examples, clearly illustrates the importance of minimizing weed seed rain and the ability to reduce an existing weed bank. Depleting the seedbank also underlies the false and stale seedbed techniques, as these deplete the emergable weed seedbank (see Section 10.8.2.1 ).
In terms of replenishing the weed seed bank, the potential is almost limitless. Many weeds can produce tens, hundreds of thousands, even millions of seeds ( Robbins et al., 1953; Salisbury, 1961; Gwynne and Murray, 1985 ), though seed production per plant is typically much lower due to competition. As an example a weed population of 1000 weed m 2 (one weed per 10 cm 2 ) and 1000 seeds per weed, represents ten billion seeds per hectare or one seed per square millimeter! While this shows the potential to rapidly refill the weed seed bank, it should not be concluded that the effects will persist for many years, rather the above research shows that it can be quickly reduced again.
Biology and Ecology of Weeds and the Impact of Triazine Herbicides
Homer M. LeBaron , Gustav Müller , in The Triazine Herbicides , 2008
Herbicides and Weed Biology
The use of triazine herbicides resulted in the control of many weed species with one application. Research showed that repeated control of weeds resulted in reductions in the weed seedbank in soil after several years. In a 6-year study in Colorado, Schweizer and Zimdahl (1984) found the number of seeds in the seedbank decreased by approximately 70% after 3 years of annual atrazine application plus interrow cultivation. Atrazine use was ceased in some plots after the first 3 years, and weeds were controlled with one or two cultivations. After 3 years of cultivation only, the weed seedbank was approximately 25 times greater than those where atrazine use and cultivation were continued. A similar study was conducted at five locations in Nebraska ( Burnside et al., 1986) . Broadleaf and grass seed density in the soil declined by 95% after a 5-year weed-free period. During the sixth year, herbicide use was ceased, and seed density increased to 90% of the original level at two of the five locations. These studies demonstrate that weed management has a great impact on the weed seedbank, resulting in a rapid decline in the seedbank when seed introductions are minimized or prevented. However, a small number of seeds of most weed species remain viable for long periods in the soil, and when weed management practices are not entirely effective, these seeds can germinate, mature, and produce enough seed to replenish the seedbank ( Buhler et al., 1998) .
Norris (1992) proposed that with proper use of herbicide and weed management technology, we can eliminate weeds from an area by preventing weeds from producing seed. He further stated that the economic threshold, defined as the pest population at which control action should be initiated in order to prevent the population from increasing to or exceeding the economic injury level, should not be adopted in weed management as it has been in entomology for insect management. Weed management must recognize long-term weed population dynamics, including the nature of the seedbank. He recommended that weed management, especially for serious problem weeds, should adopt a ‘no-seed’ threshold. This threshold implies that weeds should not be permitted to set seed. He cited several cases where this has worked in California on high-value crops where the same growers are in control of the land for many years. Norris (1999 , 2000 ) further stated that a ‘no seed’ threshold can only be successful when weed management technologies are integrated, including the use of hand labor for controlling low-weed populations that have not succumbed to other management tools.
Jones and Medd (2000) proposed that a longer-term management approach is needed to manage weed seedbanks and to determine the optimal level of intervention required for a specific weed situation. Managing seedbanks is complex because of the difficulty in preventing seed production and introduction, as well as the persistence of certain seeds in the seedbank and the high seed production potential of many weed species ( Buhler et al., 1998) . Weed seedbanks are an ever-present component of agricultural land, and resources directed to understanding, interpreting, and predicting seed germination potential can improve agricultural production. Management systems can be devised that minimize the impact of the resultant weeds.
Cousens and Mortimer (1995) confirmed that fields receiving herbicides annually for more than 20 years may be reinfested with damaging weed flora if left unsprayed, often within one or a few years.
Weed populations are never constant, but are in a dynamic state of flux due to changes in climate, environmental conditions, tillage, husbandry methods, use of herbicides, and other means of control. Weeds that were at one time of minor importance, but not controlled by certain broad-spectrum herbicides, have increased to become major problems. Reduction in tillage has sometimes led to the increased occurrence of perennial weeds and annual grasses, particularly of those species that readily establish near the soil surface and have relatively short periods of dormancy. Many perennials have increased in importance under minimal cultivation (e.g., field bindweed and Canada thistle). The occurrence of herbicide-resistant weed biotypes is also a phenomenon of increasing concern. Some research results show that large changes in the seedbank can impact weed control efficacy. Winkle et al. (1981) and Buhler et al. (1992) found large increases in weed densities reduced weed control with herbicides and mechanical practices.
Webster and Coble (1997) reported on weed shifts in major crops of the Southeastern states over a 22-year period (1974–1995) when herbicides were the major means of weed control. Sicklepod and bermudagrass had become the most troublesome weeds. The largest decreases in weed pressure were found with Johnsongrass, crabgrasses, and common cocklebur. Morningglories and nutsedges remained relatively constant. The weeds of greatest importance in soybean, peanut, and cotton are the pigweeds.
Webster and Coble (1997) listed several factors that may play an important role in the future weed species composition of cropland: (1) Herbicide-resistant weeds represent a change in the weed spectrum in some of the management systems, with almost every state having at least one reported herbicide-resistant weed. (2) Cropping systems that use fewer tillage operations may allow weeds that are unable to survive frequent disturbances (e.g., biennials and simple perennials) to invade and become problem weeds in fields. (3) A reduction of triazine herbicides used in corn and cotton weed management systems may allow previously controlled broadleaf weeds to become major weeds again. (4) The widespread use of herbicide-tolerant crops may have a further significant impact on the weed species composition.
Changes in weed species and populations also cause changes in plant diseases and insect pests since certain weeds serve as their hosts ( Bendixen et al., 1981 ; Manuel et al., 1982 ; Weidemann and TeBeese, 1990 ; Norris and Kogan, 2000) . Herbicide-resistant weed biotypes are present in our weed populations, although often at very low frequencies, even when herbicides are not used. Weed species have acquired built-in genetic adaptability to survive most control methods used against them. For example, dandelions usually develop a vertical growth habit when growing wild, but when growing in a frequently mowed lawn, more prostrate or flat-growing biotypes evolve. We should continually add to our weed control technology and keep tools available in order to address the adaptability of weeds to different control methods. For further information on the biological characteristics of weeds, including growth strategies, mimicry with crops, plasticity of weed growth, photosynthetic pathways, weed seed reservoir, and vegetative reproduction see Cousens and Mortimer (1995) and Buhler et al. (1998) .
Weeds Resistant to Nontriazine Classes of Herbicides
Homer M. LeBaron , Eugene R. Hill , in The Triazine Herbicides , 2008
Use of Modeling in Managing Herbicide Resistance
The rate of evolution of resistant weeds is based on several factors, including characteristics of the weed and herbicide, gene frequency, size and viability of the soil seedbank, weed fitness, herbicide potency, frequency and rate of application, and persistence in soil. Various attempts have been made to use modeling to determine the relative importance of these factors and to predict the probability of resistance, as well as to evaluate how to avoid, delay, or solve the problem ( Gressel and Segel, 1990 ).
Richter et al. (2002) have reviewed the use of models to evaluate the dynamics of herbicide resistance and to develop suitable anti-resistance strategies. Herbicide resistance is impacted by a high initial frequency of resistance alleles in a population, out-breeding, dominance of inheritance, a short persistence of the seed bank in the soil, and the lack of a fitness penalty for resistant versus susceptible biotypes of a weed species, along with agronomic factors having a positive influence on weed development. The occurrence of herbicide-resistant weeds in a field usually means the loss of an effective control measure. This is particularly serious if resistance develops in species for which there are few if any effective alternatives. As a rapid increase in the development of herbicides with new modes of action is not likely, and since economic and environmental conditions often will not support cultural control measures or alternative cropping systems, it is important to manage resistance wisely in order to avoid further loss of herbicides.
Using a model to maximize strategies for herbicide-resistant blackgrass, Cavan et al. (2000) gave estimates on the effectiveness of various strategy options. Based on research with a long-term model for control of blackgrass and annual bluegrass, Munier-Jolain et al. (2002) concluded that threshold-based weed management strategies can be more cost-effective than spraying every year and may enable important reductions in herbicide use. However, the highest long-term profitability was obtained for the lowest weed level threshold tested.
Müller-Schärer et al. (2000) reviewed the progress made during 1994–1999 by 25 institutions within 16 European countries on biological weed control. These efforts were aimed at control of major weed species, including common lambsquarters, common groundsel, and species of pigweed, broomrape and bindweed in major crops, including corn and sugar beet. No practical control has yet been reached for any of the five target weeds, however, the authors concluded that potential solutions have been identified.
Weed and soil management: A balancing act☆
Trevor Kenneth James , Charles Norman Merfield , in Reference Module in Earth Systems and Environmental Sciences , 2021
Weed management interacts with soils in multiple ways. The primary aim of weed management is to limit weed populations below the levels where they cause economic losses, both in current and future crops. Weed management therefore deliberately aims to reduce plant biodiversity, both in the amount and range of non-crop species, which produces a cascade of negative ecological effects, including reduced soil health. Where weed management achieves a very large reduction in weed populations bare soil can result which is at risk of erosion and other damage. Tillage (e.g. mouldboard ploughing/inversion tillage) is an integral part of weed management in many farming systems, particularly cropping systems (e.g., cereals and vegetables), and tillage’s negative impacts on soil are well documented, including in this encyclopedia. In response to the harmful effects of tillage, especially inversion tillage, on soil health, reduced/minimum tillage and no-tillage systems were developed and have been widely adopted. Changing tillage systems can have large impacts on both weed flora as well as soil health.
The soil is where the weed seedbank is situated, and particularly for therophyte weeds, managing the weed seedbank is critical for effective weed management, especially under non-chemical and integrated weed management systems ( Fig. 1 ). Soil health and function also have a large impact on the weed seedbank, as there are many organisms in soil, from microorganisms through invertebrates to vertebrates that predate on seeds, so soil management and soil health can affect how much of the weed seedbank is lost to predation. In comparison, mechanical techniques to reduce the weed seedbank, such as fallowing are highly damaging to soil due to both repeated tillage and absence of living plants, resulting in compaction, loss of structure, reduction in soil biological diversity, etc.
Fig. 1 . Weed seed banks can contain millions of seeds giving rise to large numbers of weeds, Digitaria sanguinalis (L.) Scop. uncontrolled in maize (Zea mays L.).
Integrated weed management (IWM) is the approach promoted by weed scientists as the most effective long-term and damage limiting approach to weed management. It systematically integrates physical, chemical, biological and ecological approaches using the technique that achieves optimal weed control with the fewest negative outcomes for the wider environment for any given issue. However, physical and chemical techniques are often harmful to soil, and greater use of biological and ecological techniques is required. Additionally, over reliance on chemical control can lead to herbicide resistant weeds. One such technique is cover cropping, which has many permutations and can achieve good weed management while at the same time avoiding the damage and consequences of physical and chemical techniques and even improving soil health at the same time.
System level farm tools for weed management, for example, diversified rotations, can have positive effects on soil health, e.g. the inclusion of a pasture phase in a cropping rotation, improves all aspects of soil quality and health, and reduce the soil seedbank and therefore the need to use harmful agrichemicals.
While weed management has multiple effects on soil, many negative, effective weed management is critical in farming, particularly cropping. If not controlled, weed populations can rapidly reach exceptionally high levels to the point that crop yield is dramatically reduced, even to the level of complete crop loss. Further, for any given crop or pasture there will always be many different species of weeds present and any given weed species can infest a very wide range of crops and pastures, which means weeds are the ubiquitous pests of crops and pasture. This contrasts with invertebrate pests and diseases (e.g. fungi and bacteria) of plants which are typically highly host specific, i.e., any given pest can attack only a narrow range of host species, and vice versa any given crop species is only attacked by a small range of pests and diseases. Weeds can also host crop plant pests and diseases. Effective weed management is therefore vital for successful agriculture and horticulture.
Weed management therefore interacts with soil in many different ways, and while often having negative effects, there are many techniques available to help ameliorate negative effects and increase positive effects. Weed management is therefore a balancing act, between achieving sufficient weed control for good crop yield and quality, while minimizing negative impacts on soil and the rest of the farm environment.
Weeds of farm crops
Some weed species are able to produce thousands of seeds per plant. Examples of prolific seed producers include corn poppies and mayweed species. The seed reservoir (weed seed bank) in some soils can be as high as 40 000/m 2 . Not all the seed produced in one year will germinate the next year; the percentage emergence may only be around 2–6% of the weed seedbank. Many species have some sort of seed dormancy mechanism that has to be broken before they will germinate. Once dormancy has been broken environmental conditions must also be correct for germination; this accounts for some of the variation in weed populations between years. Losses of seed and seed viability are taking place all the time. Depth of burial in the soil, number and type of cultivations, soil type and weed species affect the rate of decline. The seed viability of some species such as fumitory, charlock, black bindweed, wild oats and corn poppy declines very slowly compared with the rapid decline of some grasses such as barren brome.
Preventive Weed Management in Direct-Seeded Rice
Adusumilli N. Rao , . David E. Johnson , in Advances in Agronomy , 2017
2.5.2 Stimulating Fatal Germination With Crop and Cover Crop Rotations
Rotational crops (including cover crops) often entail the application of practices which stimulate germination (e.g., tillage and irrigation) as well as those that kill emerged seedlings (e.g., cultivation and herbicides). Rotational crops may result in the germination and death of rice weeds in a manner analogous to the stale seedbed approach described earlier.
As with a stale seedbed, the success of rotational crops in reducing the rice weed seedbank depends on an appropriate stimuli being applied at the right time (when seeds are relatively nondormant), as well as on the use of effective postemergence termination methods. Likewise, the efficacy of rotational crops in promoting fatal germination is likely to be the greatest for weeds with limited dormancy and in rotational crops for which weed management is relatively easy and inexpensive. Indeed, if rice weed seeds are stimulated to germinate in rotational crops and are not effectively terminated, they may exacerbate weed problems through reproduction.
In the rice–wheat rotation of India, the inclusion of mungbean during the fallow period between wheat harvest and rice planting resulted in 84% and 40% reduction in the population of D. aegyptium in the subsequent rice crop under ZT and CT systems, respectively ( Fig. 2 ). This was because of the greater emergence of this weed species following irrigation during the mungbean cropping, followed by effective termination using nonselective herbicides (in ZT) and shallow tillage (in CT).
Fig. 2 . Effects of crop rotation (by including mungbean) and tillage in the rice–wheat rotation system of India on the cumulative emergence of Dactyloctenium aegyptium during fallow/mungbean period and during the subsequent direct-seeded rice crop (Kumar et al., unpublished data).
In temperate cropping systems, cover crops have been evaluated for their potential to promote the fatal germination of weed seeds. For example, Mirsky et al. (2010) reported declines in weed seedbanks by encouraging fatal germination associated with soil disturbance in cover crop treatments. Cover crops stimulated weed seed germination and the germinated weeds were either suppressed by the cover crop or controlled by subsequent tillage and preempted weed seed rain. The stimulative effect of certain cover crops has been proven particularly helpful in the management of parasitic weeds. In upland DSR fields in East Africa, green manure ( Crotalaria ochroleuca, M. invisa, and Cassia obtusifolia) exhibited a potential to induce the suicidal germination of S. asiatica ( Kayeke et al., 2007 ). The cover crops in this case served as a false-host by stimulating the germination of Striga without providing conditions necessary for survival.